Polyolefins are used throughout modern society. Some varieties are inexpensive thermoplastic polymers employed in wide varieties of applications, the articles of which include, for example, films, fibers, such as spunbonded and melt blown fibers, fabrics, such as nonwoven fabrics, and molded articles. Polymer selection for any one particular application depends, in part, on the polymer's properties, the fabrication mode or manufacturing process, the final article and its intended uses. Examples of some of these properties include density, molecular weight, molecular weight distribution, melting temperature, melt strength, and melt flow rate.
Polymer properties are generally dependent upon the conditions present during polymerization. One such condition is the catalyst. In some instances, while catalyst selection is an important reaction parameter, changing other variables in the presence of the same catalyst produces different polymer properties. For example, adding hydrogen to a catalyzed polymerization reaction may increase the catalyst activity. Catalyst activity can be measured by the increase or decrease in the amount of polymer produced during a measured time interval by a measured amount of catalyst. Generally, an increase in catalyst activity results in an increase in polymer production. Producing more polymer with the same catalyst amount or using less catalyst to produce the same polymer amount provides a commercial advantage.
There are many instances where hydrogen addition not only increases polymer amount, but also increases polymer melt flow rate (MFR). Many manufacturing processes have specific, if not strict, polymer MFR requirements. For example, High-MFR (low molecular weight) polymers are suitable for nonwoven applications because melting and handling the molten polymer are common steps in converting the polymer into a finished article, while low-MFR (high molecular weight) polymers are required for foaming and thermoforming applications.
In addition to MFR and molecular weight, molecular weight distribution (MWD) is an important polymer property. Similarly, catalyst selection is also an important parameter. For example, conventional multisite catalysts produce polymers with broad MWD, which are easy to process in fabrication, while polymers made with single site catalysts generally have narrow MWD with higher toughness and clarity. Certain catalysts and particularly certain metallocene catalysts are suitable for producing polymers of a particular MFR. Different polymer end uses require different MFR. This frequently means that a polymer manufacturer must change catalysts when targeting a different end use—an expensive and time consuming proposition.
Incorporating α,ω-diene molecules into the polymerization reaction alters the apparent MFR response of a catalyst. Thus, diene incorporation allows a manufacturer to target different polymer end uses while short cutting around the catalyst change.
Incorporating α,ω-dienes in the polymerization process has been used to introduce branching into the polymer, which improves the polymer's melt properties. U.S. Pat. No. 5,670,595 describes diene-modified polymers made with metallocene catalysts. But diene-modified polymers usually contain high molecular weight components due to the chemistry of diene-induced LCB-polymer formation. These components sometimes degrade LCB-polymer properties.
Diene incorporation allows polymer property and catalyst response tuning. But what is needed is a diene-incorporation tool that provides for such tuning while avoiding the runaway crosslinking that degrades diene-modified-polymer properties. Moreover, a straightforward way to select polymer molecular weight distribution without changing catalysts until now, remains unavailable.
For additional background, see also WO 99/45046; WO 99/45049; U.S. Pat. No. 4 306 041 A; WO 98/02471 A; U.S. Pat. No. 3,718,632 A; WO 00/11057A; U.S. Pat. No. 5,670,595 A; WO 99/45046 A; WO 99/45049 A; U.S. Pat. No. 4,306,041 A; WO 98/02471 A; U.S. Pat. No. 3 718 632 A; WO 00/11057 A; and U.S. Pat. No. 5,670,595 A.